Method of reducing carbon monoxide formation in catalytic heaters and burners and improved catalytic structure for use therewith

ABSTRACT

This disclosure is concerned with reducing carbon monoxide formation in catalytic heaters and burners employing preferably platinum catalytic surfaces to which hydrocarbon gaseous fuel is diffused, and which are treated with carbonate, acetate, borate, sulphate, nitrate, hydroxide, and chloride salts of certain alkali earth metals.

United States Patent [191 Petrow METHOD OF REDUCING CARBON MONOXIDE FORMATION IN CATALYTIC HEATERS AND BURNERS AND IMPROVED CATALYTIC STRUCTURE FOR USE THEREWITH [75] Inventor: Henry G. Petrow, Cambridge, Mass.

[73] Assignee: Prototech Company, Newton, Mass.

[22] Filed: Dec. 29, 1972 [21] Appl. No.: 319,170

[52] US. Cl 431/7, 252/476, 252/477 R [51] Int. Cl. F23b 7/00 [58] Field of Search 431/328, 329, 7, 268;

[56] References Cited UNITED STATES PATENTS 3,291,564 12/1966 Kearby ..252/476X [451 Jan. 21, 1975 3,590,806 7/1971 Locke 431/328 X 3,615,216 10/1971 Aldridge 252/476 X 3,649,566 3/1972 Hayes et al. 252/476 X 3,658,927 4/1972 Crain et al. 252/476 X 3,697,447 10/1972 Bettinardi 431/328 X 3,755,556 8/1973 Aldridge et a1. 252/476 X Primary Examiner-Charles J. Myhre Assistant Examiner-William C. Anderson Attorney, Agent, or Firm-Rines and Rines; Shapiro and Shapiro [57] ABSTRACT 10 Claims, N0 Drawings METHOD OF REDUCING CARBON MONOXIDE FORMATION IN CATALYTIC HEATERS AND BURNERS AND IMPROVED CATALYTIC STRUCTURE FOR USE THEREWITH Thepresent invention relates to methods of reducing carbon monoxide formation in catalytic heaters and burners and the like, and to improved catalytic structures for use in the practice of such methods; the invention being more particularly directed to the operation of catalyst-provided mats employed with hydrocarbon gases diffused therein for such purposes as space heating applications, including recreational, domestic, agricultural and industrial heaters and burners.

Structures of the space heater type employing such catalytic reactions are described, for example, in The Catalytic Heater, Appliance Engineer, October, 1972, p. 32. Generally, a fiber or other mat is coated with an appropriate catalyst as, for example, platinum, and is fitted in an assembly wherein hydrocarbon gaseous fuel, such as methane, propane, or butane, or gasoline or the like, is diffused through the mat to reach the catalyst surface that is in contact with the air. The application of a flame to ignite the system causes the raising of the temperature of the mat assembly sufficiently to sustain flameless catalytic combustion of the fuel, efficiently generating the required heat. As an example, propane or methane combustion systems of this type are operated so as to generate approximately 75 BTU per square inch per hour of catalyzed surface.

Unfortunately, the pattern of combustion in such systems is not uniform, such that certain areas, including those closer to the source of diffusing fuel, burn hotter than other areas, in turn creating a suction effect wherein the fuel tends to diffuse more rapidly to the areas of faster consumption. This effect becomes cumulative in operation and deleteriously results in the excessive formation of carbon monoxide. It has been found, indeed, that carbon monoxide levels as high as 2,000 to 3,000 parts per million and greater have been frequently inherently generated with certain very active platinum catalysts; and even as high as 400 to 500 parts per million result in commercial burners of this type presently on the market. While there presently are no codified regulations relative to safe carbon monoxide levels in the operation of these burners, the industry prefers levels (expressed on an air-free basis) of 200 parts per million or less; and, indeed, if at all possible, levels less than 100 parts per million.

The problems involved in reducing carbon monoxide formation in such combustion systems are, however, quite different from those heretofore encountered in catalytic burning systems of other types, where there is the opportunity actively to mix air with the fuel prior to combustion to insure the desired degree of combustion. In the case of the space heaters and the like, above described, however, the fuel is merely diffused into the catalyst coated mat, and it is only the external air surrounding the mat that is available for the combustion process. This problem does not thus lend itself to solution by the mixing and related prior art techniques in other types of catalytic combustion systems and processes.

In accordance with a discovery underlying the invention, it has been found possible to control the carbon monoxide formation through the rather critical use of certain additives to the system that fortuitously and unexpectedly have been found not only to make the catalytic combustion process remarkably complete, but to do so without impairing the catalytic properties, life, and other combustion characteristics of the system. It is therefore an object of the present invention to provide a new and improved method of reducing carbon monoxide formation in catalytic heaters and burners and the like of the above-described type, and novel catalytic structures particularly adapted for use in the carrying out of such method.

Still a further object is to provide a new and improved catalytic method and structure that is particularly adapted to render more uniform and complete the catalytic combustion of hydrocarbon gases and the like.

An additional object is to provide a new and improved method of and structure for reducing carbon monoxide formation and improving the combustion efficiency of catalytic heaters and burners and the like, that are of more general application in catalytic processes, as well.

In summary, this end is attained in accordance with a method of reducing carbon monoxide formation in the catalytic combustion of a hydrocarbon gaseous fuel, that comprises, diffusing a hydrocarbon gaseous fuel into a mat, disposing a catalytic surface exposed to air on an outer surface of the mat, and applying to said surface salts selected from the group consisting of carbonates, acetates, borates, sulphates, nitrates, hydroxides and chlorides of alkali earth metals selected from the group consisting of Cs, Rb, K and Na.

The invention will now be described in connection with its important and preferred application, as an illustrative example, to the before-mentioned space heaters and the like, embodying fiber or similar mats provided with surface-coated catalysts and operated with hydrocarbon gaseous fuels diffused into the mat. In preferred illustrations, the mat may be formed of alumino-silicate fibers surface-coated with a platinum catalyst and employing, for example, propane as the fuel. If the beforedescribed mat is coated with an acceptable platinum catalyst and operated with propane fuel, the carbon monoxide levels almost invariably are in excess of the desired maximum of 200 parts per million. The bulk of the burning occurs in close proximity to the point of fuel injection in the rear of the mat. This is obvious when inspecting and operating the mat in darkness. Certain portions of the mat in close proximity to fuel injection burn with an intense cherry red, where other parts of the mat, even in darkness, show no noticeable glowing indicative of high temperature processes. Temperature variations between the very hot parts of the mat and the cooler portions can be in excess of 200C; that is, the cooler areas will register temperatures as low as 250C and some of the very hot areas as high as 500550C. In extremely adverse situations, portions of the mat actually are inoperative, being below the catalytic combustion ignition temperature (of the order of 220C in the case of propane) and result in the leakage of uncombusted gas.

Levels of carbon monoxide will vary depending upon the amount of catalyst employed and the nature of the platinum compound used as a catalyst. For example, a series of commercially available catalytic burners was tested and found to yield carbon monoxide levels in the range of 250 to 575 parts per million, with the average value being approximatley 330 parts per million. Measures have been explored to determine what techniques, such as varaitions in platinum catalysts, methods of distribution of the gas, thermal treatment of the applied catalyst, and the use of additives, etc., would be efficacious in reducing carbon monoxide emanations to safe and acceptable levels. With present-day commercial mats, thermal pre-treatment even at temperatures such as 650-700C was found to be ineffective adequately to solve the problem. While it was found that if fundamental changes in the design of the burner and specifically the gas distribution system were made, carbon monoxide levels could be reduced, manufacturers of burner assemblies were not receptive to this redesign approach.

The possibility of using additives, such as co-catalysts (as, for example, platinum-palladium mixtures and the like) were studied, but were found to produce no substantial improvement in the carbon monoxide reduction. Other metals with known catalytic activity such as iron, copper, manganese, silver and the like, also failed to reduce permanently carbon monoxide levels in these applications. Since it was apparent as a result of these researches that co-catalysts do not appear to be a fruitful technique for the resolution of this problem, additives which have no known catalytic properties, at least for oxidation of hydrocarbons, and that are not known to poison platinum, were also examined, including compounds of calcium, barium, magnesium, titanium and lithium; but all such were found to be without substantial improvement effect on reduction of carbon monoxide formation.

Quite surprisingly, however, especially in the light of the ineffectual performance of lithium, the salts of alkali metals of sodium, potassium, rubidium and cesium were found markedly to lower the amount of carbon monoxide generated; and at the same time fortuitously gave no impairment of the essential properties of the mat, such as its durability and efficiency of fuel combustion. Additional noticeable advantages in such use, apart from the reduction of carbon monoxide levels, were a surprisingly more uniform pattern of burning and generally a somewhat improved efficiency of fuel combustion.

Another startling property of these particular alkali metal compounds was that they did not have, in terms of equal molecular quantities added, the same overall effectiveness. Cesium and rubidium salts were approximately three times more effective than potassium, which in turn was approximately twice as effective as sodium. A system that would require 6 moles of sodium per mol of platinum catalyst would provide approximately the same result with only three moles of potassium salt and only one mol of either cesium or rubidium salt. Since cesium and rubidium salts are rare and expensive and since sodium salts are the least effective, potassium salts were chosen as the preferred material.

While it is not required that a precise theory of operation be known, and the present invention is not to be construed as dependent upon the same, it being sufficient to describe the steps and structures that have been found in practice to produce the novel results of the invention, it is believed probable that these results may occur as a result of the following phenomena.

First of all, it was established that it was, indeed, the alkali metal that was effective and not the anion that accompanies the alkali metal in the applied compound. Equivalentresults were obtained, for example, regardless of whether potassium was added as the sulphate, carbonate, nitrate, chloride, acetate, hydroxide or the borate. This was equally true in the case of sodium, cesium and rubidium.

Secondly, it was demonstrated that this property did not arise from an alteration of the surface area or of some other fundamental properties of platinum itself, in that this beneficial effect could be obtained either by mixing the desired alkali metal compound with the platinum catalyst at the time at which the surface of the mat was catalyzed, or by applying the desired alkali metal compound on a catalyzed mat even surfaces which had hundreds of hours of operation such that the platinum presumably had achieved well-defined and reasonably permanent structure.

A basic clue as to why these alkali metals are operative appears to reside in the noted fact that cesium is the most effective and lithium is largely ineffective, and with a pattern of decreasing effectiveness as one moves through the alkali metal family from cesium to lithium. While in any chemical family there is a periodic change of properties up or down in atomic number, one important property within a given family is a change in the ionization potential. Among the alkali metals, cesium is most easily ionized, and lithium is the most difficult to ionize. It is thus believed that the mechanism in the reduction of carbon monoxide and the production of more uniform burning and a somewhat increased lightoff time, achieved with the present invention, is a kinetic effect in the burning process wherein electrons thermally generated from the low-ionization potential alkali metals act as free radical scavengers or interrupters, thereby giving a more orderly and less chaotic burning process.

This discovery and use, moreover, is to be distinguished from the prior use of, for example, sodium hydroxide in nickel-deactivating processes for reforming and the like, wherein a change in acidity is effected to cause catalytic deactivation, as is well known and practiced in the art, and where all alkali metal hydroxides are equally effective, and neutral alkali salts are all ineffective.

It is this imposed uniformity of the burning process, indeed, that appears to be responsible for the reduction in the carbon monoxide content, in that the opportunity for depleting to an undesirable level the available oxygen at any point on the catalytic surface is diminished. The result is the creation of a new surface with better capabilities for the uniform combustion of the fuel, so as to minimize carbon monoxide formation.

As a first example of the efficacious nature of the invention, a platinum catalyst was applied at the level of 2 milligrams of platinum per square inch of surface to an alumina-silicate fiber mat. Propane was used as the fuel, diffused into the mat with the propane fuel feed rate being such as to yield, under conditions of total combustion, BTUs per hour per square inch of surface. The carbon monoxide generated under these conditions had a concentration in the release combustion products of about 600 parts per million.

When the identical structure was used, except that before application of the catalyst to the fiber mat, 3 moles of potassium borate per mole of platinum were incorporated in the platinum catalyst solution, the resulting mat, when similarly operated with propane as a fuel, yielded only about 25 parts per million of carbon monoxide.

As further examples, precisely the same tests were performed with separate platinum catalytic structures to which were applied, separately, each of potassium carbonate, potassium acetate, potassium nitrate, potassium hydroxide and potassium chloride. In all cases, the potassium-to-platinum mole ratio remained 3-1. Within possible experimental variations, each of these salts gave carbon monoxide reduction and uniform burning results comparable to those observed with potassium borate. As another example, the untreated catalytic mat described in the first example, after having been in operation for about 200 hours, and then producing carbon monoxide at a level of about 400 parts per million, was treated with potassium borate in the same amount as previously described. The carbon monoxide level was thereupon reduced to about 50 parts per million.

In the first example, supra, moreover, the levels of unburned propane were also determined. With the untreated platinum-coated mat, 38 percent of propane was found in the release gases. For the mat where the potassium borate had been added, the propane level was strikingly lower; about 0.15 percent.

As additional illustrations, the test described in the first example was repeated using each of cesium borate, rubidium borate and sodium borate, all providing substantial carbon monoxide reduction and more uniform burning. These studies demonstrated that for effects as high as those attained with potassium borate, one-third as much cesium and rubidium salt was required, and twice as much sodium.

In further tests of the same structure, the amount of potassium borate was varied from about /2 to over 12 moles per mole of platinum catalyst. In the case of /2 mole, the carbon monoxide level, while being substantially reduced, was not reduced quite sufficiently to meet the before-described minimum levels. When more than about 12 times as much potassium borate was used, the carbon monoxide levels remained low, but the light-off characteristics of the mat became sluggish, requiring 2 to 3 times the time for full ignition. This same range of additive concentration was found for the other alkali earth salts above described; the amount of the particular alkali metal salt applied being rather critical and depending somewhat upon the particular metal used. Though the determined general range is of the order of from about I to 12 moles per mole of applied platinum, with potassium, the preferred range was found to be about 2-9 moles per mole of catalyst; for sodium, about 4-12; and for rubidium and cesium, about 1-3, when employing the platinum catalyst described below.

Potassium borate is additionally preferred, though the other salts may, of course, be employed, because it has only a weakly alkaline ph (around 9), and it will not absorb CO at high temperatures to form the more alkaline potassium carbonate. It is a quite stable entity .and there is no indication that the borate can be reduced to the elemental state such that it could possibly alloy or otherwise affect the nature of the platinum.

While the invention has been found to be extremely efficacious with plaintum catalytic surfaces, these surfaces may assume somewhat different natures. In a preferred embodiment, used in the first-named tests before described, the platinum may be very finely dispersed with particle size substantially all of the order of from 15 to 25 Angstroms, being preferably formed as the colloid from the decomposition of a platinum salt containing two moles of per platinum atom as described in co-pending Petrow et al U.S. application, Ser. No. 153,824, filed 6/16/72 and now abandoned, for Finely Particulated Colloidal Platinum, Compound and Sol for Producing the Same, and Method of Preparation. Other types of catalytic structurewith which the invention has been found to work, however, are the commercial platinum catalysts now being marketed by the Matthey-Bishop Corporation for propane, and mats catalyzed with palladium, formed by the decomposition of palladium nitrate solution applied to the mat, as above described.

Further modifications will also occur to those skilled in this art and all such are considered to fall within the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

l. A method of reducing carbon monoxide formation in the catalytic combustion of a hydrocarbon gaseous fuel, that comprises, diffusing a hydrocarbon gaseous fuel into a mat, disposing a catalytic surface exposed to air on an outer surface of the mat, and applying to said surface salts selected from the group consisting of carbonates, acetates, borates, sulphates, nitrates and chlorides of alkali earth metals selected from the group consisting of Cs, Rb, K and Na.

2. A method as claimed in claim 1 and in which the amount of applied metal of the metal salt is in the range of from about 1 to 12 moles per mole of catalyst.

3. A method as claimed in claim 2 and in which said catalytic surface comprises platinum.

4. A method as claimed in claim 3 and in which said metal salt is potassium borate applied in an amount in the range of from about 2 to 9 moles of potassium per mole of platinum catalyst.

5. A carbon-monoxide-formation-reducing catalytic structure for combusting a hydrocarbon gaseous fuel having, in combination, a mat-like structure for receiving hydrocarbon fuel, such structure being provided with an external platinum-containing surface exposed to the air, and applied to said surface, salts selected from the group consisting of carbonates, acetates, borates, sulphates, nitrates, hydroxides and chlorides of alkali earth metals selected from the group consisting of Cs, Rb, K and Na.

6. A carbon-monoxide-formation-reducing catalytic structure as claimed in claim 5 and in which the amount of the metal of the metal salt applied is in the range of from about 1 to 12 moles per mole of platinum catalyst.

7. A carbon-monoxide-formation-reducing catalytic structure as claimed in claim 5 and in which said matlike structure comprises an alumina-silicate fiber structure.

8. A carbon-monoxide-formation-reducing catalytic structure as claimed in claim 5 and in which said platinum is in the form of particles formed from decomposition and then reduction of a platinum salt containing two moles of 80;; per platinum atom.

9. A method of reducing carbon monoxide formation in the catalytic combustion of a hydrocarbon gaseous fuel. that comprises, diffusing a hydrocarbon gaseous fuel into a mat, disposing a catalytic surface exposed to air on an outer surface of the mat, and providing along said surface during combustion free electrons from low-ionization potential alkali metals to scavenge radiwith an external palladium-containing surface exposed to the air, and applied to said surface, salts selected from the group consisting of carbonates, acetates, borates, sulphates, nitrates, hydroxides and chlorides of alkali earth metals selected from the group consisting of Cs, Rb, K and Na. 

2. A method as claimed in claim 1 and in which the amount of applied metal of the metal salt is in the range of from about 1 to 12 moles per mole of catalyst.
 3. A method as claimed in claim 2 and in which said catalytic surface comprises platinum.
 4. A method as claimed in claim 3 and in which said metal salt is potassium borate applied in an amount in the range of from about 2 to 9 moles of potassium per mole of platinum catalyst.
 5. A carbon-monoxide-formation-reducing catalytic structure for combusting a hydrocarbon gaseous fuel having, in combination, a mat-like structure for receiving hydrocarbon fuel, such structure being provided with an external platinum-containing surface exposed to the air, and applied to said surface, salts selected from the group consisting of carbonates, acetates, borates, sulphates, nitrates, hydroxides and chlorides of alkali earth metals selected from the group consisting of Cs, Rb, K and Na.
 6. A carbon-monoxide-formation-reducing catalytic structure as claimed in claim 5 and in which the amount of the metal of the metal salt applied is in the range of from about 1 to 12 moles per mole of platinum catalyst.
 7. A carbon-monoxide-formation-reducing catalytic structure as claimed in claim 5 and in which said mat-like structure comprises an alumina-silicate fiber structure.
 8. A carbon-monoxide-formation-reducing catalytic structure as claimed in claim 5 and in which said platinum is in the form of particles formed from decomposition and then reduction of a platinum salt containing two moles of SO3 per platinum atom.
 9. A method of reducing carbon monoxide formation in the catalytic combustion of a hydrocarbon gaseous fuel, that comprises, diffusing a hydrocarbon gaseous fuel into a mat, disposing a catalytic surface exposed to air on an outer surface of the mat, and providing along said surface during combustion free electrons from low-ionization potential alkali metals to scavenge radicals from the hydrocarbon fuel being catalytically combusted, and thus to slow down and more orderly burn the fuel.
 10. A carbon-monoxide-formation-reducing catalytic structure for combusting a hydrocarbon gaseous fuel having, in combination, a mat-like structure for receiving hydrocarbon fuel, such structure being provided with an external palladium-containing surface exposed to the air, and applied to said surface, salts selected from the group consisting of carbonates, acetates, borates, sulphates, nitrates, hydroxides and chlorides of alkali earth metals selected from the group consisting of Cs, Rb, K and Na. 